1. Field of the Invention
The present invention relates to a surface-emitting light-emitting diode (SLED), and more particularly, to an SLED having a low operating voltage and the high light emission.
2. Description of the Related Art
In general, the external quantum efficiency of a light-emitting diode (LED) does not exceed 10% in the case of a highly efficient LED. Such figures are very low values considering that the efficiency of a general laser diode (LD) is over 30% to nearly 50%.
The internal quantum efficiency that is the efficiency in converting a carrier (an electron-hole) injected into an active layer of a light generation layer and combined with each other to light is usually over 90% for both LEDs and LDs. Thus, it can be easily seen that the difference in the external quantum efficiencies between LEDs and LDs results from the difference in structure between two devices.
The internal quantum efficiencies of LEDs and LDs are usually reduced by various phenomenons occurring inside a semiconductor, for example, optical absorption by the active layer, loss of free carriers, absorption at defects. In LDs, the stimulated and emitted light having directivity is aligned in a particular direction according to the given waveguide structure and meets the face of a laser beam nearly perpendicularly. Thus, the light emitted to the outside of the device or the light reflected by a surface of the face travels along the waveguide to stimulate the active layer.
However, the LED uses spontaneous emission having no particular directivity in which the light generated without a specific waveguide structure proceeds in all directions in the active layer. Here, as shown in FIG. 1, when the light is propagated through a semiconductor having a designated refractive index ns passes through another medium having a different refractive index na, for example, to the air, the photon incident on the boundary surface m between two media, i.e., the surface of a semiconductor substrate, at an angle of xcex82 greater than the critical angle xcex8c is output to the outside of the semiconductor substrate according to Snell""s law. However, the photon incident at an angle of xcex81 less than the critical angle xcex8c is reflected by the boundary surface m to the inside of the semiconductor. The reflected photon may be absorbed by a crystal defect such as a dislocation in the semiconductor or absorbed by the active layer while passing therethrough. The remaining photons may pass through another boundary surface to be output to the outside. The photons failing to be output through another boundary surface may be reflected at the surface and returned to the inside of the semiconductor. The photons returned to the inside of the semiconductor may subsequently be absorbed therein. Otherwise, the photons repeat escaping through another surface. In doing so, some photons are neither absorbed by the semiconductor nor escape therefrom and endlessly travel inside the semiconductor, which is referred to as photon recycling, due to a particular output angle thereof and the symmetrical hexahedral structural feature of the semiconductor device. This phenomenon is one reason why the LEDs has a lower external quantum efficiency than the LD (refer to J. Appl. Phys, Vol. 37 (1998) 5990 part 1, No. 11, Nov. 15, 1998, Japan). The above problem is common to LEDs applied not only to GaN-based semiconductor but also to all composition semiconductors such as existing GaAs or GaP. The same problem occurs in a SLED using the light emitted along a normal line of a semiconductor layer.
FIG. 2 is a sectional view showing the stacked structure of an example of a conventional SLED having a heterojunction structure. A buffer layer 2 is formed on a substrate 1. A stepped n-contact layer 3 is formed, having a lower planarized portion at one side thereof on which an n-ohmic metal layer 9 is formed. On the n-contact layer 3, an n-lower clad layer 4, an active layer 5, a p-upper clad layer 6, a p-contact layer 7, and a translucent p-ohmic metal layer 8 are sequentially deposited. A wire bonding pad 11 is partially formed on the upper surface of the translucent p-ohmic metal layer 8.
In the above structure, in the case of a diode for blue light, the substrate 1 is generally formed of sapphire, the buffer layer 2 is formed of GaN or AlN, and two contact layers 3 and 7 are formed of n-GaN and p-GaN. The two clad layers 4 and 6 are formed of n-GaN and p-GaN or n-AlGaN and p-AlGaN, and the active layer 5 is formed of GaN and InGaN.
When power is supplied through both the ohmic contact layers 8 and 9, and current flows in the active layer 5 which is the center portion of a heterojunction structure, light is emitted from the active layer 5 due to re-combination of electrons and holes as described earlier. Here, light is emitted in all directions, and most of the light passing through the semiconductor layers disposed on and below the active layer is absorbed due to defects of the semiconductor layers. As a result, a considerably reduced amount of light passes through the translucent p-ohmic contact layer 8, and light emitted along the edge of the active layer 5 is not reduced much.
Accordingly, a SLED having the above structure is required to increase the amount of light emitted through the p-ohmic contact layer 8. Also, the drawback of a semiconductor device using III-group nitride matter is that a high operating voltage i required. The operating voltage of LED is determined by p-type contact resistance, n-type contact resistance, the resistance of a semiconductor layer where current flows, and voltage drop in the active layer. Generally, as the band gap energy of a semiconductor increases, doping becomes difficult and accordingly series resistance increases. Also, the voltage drop increases as the band gap energy increases. However, what is mattered in the GaN-based device is that p-ohmic cannot be made completely as there is not a metal having enough work function to make ohmic contact with p-GaN. Thus, the main reason for the high operating voltage of the GaN device is that the ohmic contact resistance is high.
To overcome the above problem, it is an objective of the present invention to provide a surface-emitting light-emitting diode having increased light emission.
It is another objective of the present invention to provide a surface-emitting light-emitting diode in which an ohmic contact resistance is reduced, to lower the current used and thus the amount of heat generated, thereby providing improved durability.
Accordingly, to achieve the above objectives, there is provided a surface-emitting light-emitting diode which comprises: a substrate; a light generating layer comprising an active layer for generating light, and an upper clad layer and a lower clad layer formed on and below the active layer, respectively; a lower contact layer formed between the light generating layer and the substrate; a buffer layer formed between the lower contact layer and the substrate; a lower ohmic metal layer which ohmically contacts one side of the lower contact layer; an upper contact layer formed on the light generating layer and having an uneven surface portion that is substantially sinusoidal; and a light transmissive upper ohmic metal layer formed on the upper contact layer. In the above surface-emitting light-emitting diode, the active layer is composed of GaN or AlN, the lower contact layer is composed of doped n-GaN, the upper contact layer is composed of doped p-GaN, and the substantially sinusoidal uneven surface portion increases the contact area between the upper contact layer and light transmissive upper ohmic metal layer for reducing the contact resistance therebetween.